Less hacky.

code/controllers/subsystem/pai.dm

@@ -7,7 +7,6 @@ SUBSYSTEM_DEF(pai)
 	var/ghost_spam = FALSE
 	var/spam_delay = 100
 	var/list/pai_card_list = list()
-	var/list/mob_holders_by_chassis = list()
 
 /datum/controller/subsystem/pai/Topic(href, href_list)
 	if(href_list["download"])

code/datums/elements/mob_holder.dm

@@ -5,10 +5,10 @@
 	var/alt_worn
 	var/right_hand
 	var/left_hand
-	var/can_head
+	var/inv_slots
 	var/proctype //if present, will be invoked on headwear generation.
 
-/datum/element/mob_holder/Attach(datum/target, _worn_state, _alt_worn, _right_hand, _left_hand, _can_head = FALSE, _proctype)
+/datum/element/mob_holder/Attach(datum/target, _worn_state, _alt_worn, _right_hand, _left_hand, _inv_slots = NONE, _proctype)
 	. = ..()
 
 	if(!isliving(target))
@@ -18,7 +18,7 @@
 	alt_worn = _alt_worn
 	right_hand = _right_hand
 	left_hand = _left_hand
-	can_head = _can_head
+	inv_slots = _inv_slots
 	proctype = _proctype
 
 	RegisterSignal(target, COMSIG_CLICK_ALT, .proc/mob_try_pickup)
@@ -54,7 +54,7 @@
 					"<span class='userdanger'>[user] picks you up!</span>")
 	to_chat(user, "<span class='notice'>You pick [src] up.</span>")
 	source.drop_all_held_items()
-	var/obj/item/clothing/head/mob_holder/holder = new(get_turf(source), source, worn_state, alt_worn, right_hand, left_hand, can_head)
+	var/obj/item/clothing/head/mob_holder/holder = new(get_turf(source), source, worn_state, alt_worn, right_hand, left_hand, inv_slots)
 	if(proctype)
 		INVOKE_ASYNC(src, proctype, source, holder, user)
 	user.put_in_hands(holder)
@@ -78,7 +78,7 @@
 	w_class = WEIGHT_CLASS_BULKY
 	var/mob/living/held_mob
 
-/obj/item/clothing/head/mob_holder/Initialize(mapload, mob/living/target, worn_state, alt_worn, right_hand, left_hand, can_head = FALSE)
+/obj/item/clothing/head/mob_holder/Initialize(mapload, mob/living/target, worn_state, alt_worn, right_hand, left_hand, inv_slots = NONE)
 	. = ..()
 
 	if(target)
@@ -93,8 +93,7 @@
 		lefthand_file = left_hand
 	if(right_hand)
 		righthand_file = right_hand
-	if(!can_head)
-		slot_flags &= ~SLOT_HEAD
+		slot_flags = slots
 
 /obj/item/clothing/head/mob_holder/proc/assimilate(mob/living/target)
 	target.setDir(SOUTH)

code/modules/mob/living/carbon/monkey/monkey.dm

@@ -43,7 +43,7 @@
 
 /mob/living/carbon/monkey/ComponentInitialize()
 	. = ..()
-	AddElement(/datum/element/mob_holder, "monkey", null, null, null, TRUE)
+	AddElement(/datum/element/mob_holder, "monkey", null, null, null, SLOT_HEAD)
 
 
 /mob/living/carbon/monkey/Destroy()

code/modules/mob/living/silicon/pai/pai_shell.dm

@@ -43,12 +43,6 @@
 	set_light(0)
 	icon_state = "[chassis]"
 	visible_message("<span class='boldnotice'>[src] folds out its holochassis emitter and forms a holoshell around itself!</span>")
-	if(possible_chassis[chassis])
-		var/datum/element/mob_holder/ele = SSpai.mob_holders_by_chassis[chassis]
-		if(ele)
-			current_mob_holder = ele.Attach(src)
-		else
-			current_mob_holder = SSpai.mob_holders_by_chassis[chassis] = AddElement(/datum/element/mob_holder, chassis, 'icons/mob/pai_item_head.dmi', 'icons/mob/pai_item_rh.dmi', 'icons/mob/pai_item_lh.dmi', TRUE)
 	holoform = TRUE
 
 /mob/living/silicon/pai/proc/fold_in(force = FALSE)
@@ -104,6 +98,11 @@
 			dynamic_chassis = choice
 	resist_a_rest(FALSE, TRUE)
 	update_icon()
+	if(possible_chassis[chassis])
+		current_mob_holder = AddElement(/datum/element/mob_holder, chassis, 'icons/mob/pai_item_head.dmi', 'icons/mob/pai_item_rh.dmi', 'icons/mob/pai_item_lh.dmi', SLOT_HEAD)
+	else
+		current_mob_holder?.Detach(src)
+		return
 	to_chat(src, "<span class='boldnotice'>You switch your holochassis projection composite to [chassis]</span>")
 
 /mob/living/silicon/pai/lay_down()

code/modules/mob/living/simple_animal/friendly/lizard.dm

@@ -26,7 +26,7 @@
 
 /mob/living/simple_animal/hostile/lizard/ComponentInitialize()
 	. = ..()
-	AddElement(/datum/element/mob_holder, "lizard", null, null, null, TRUE) //you can hold lizards now.
+	AddElement(/datum/element/mob_holder, "lizard", null, null, null, SLOT_HEAD) //you can hold lizards now.
 
 /mob/living/simple_animal/hostile/lizard/CanAttack(atom/the_target)//Can we actually attack a possible target?
 	if(see_invisible < the_target.invisibility)//Target's invisible to us, forget it